CARPINTERIA, Calif./LOS ANGELES (Reuters) -
Firefighters in Southern California are slowly gaining control of one of
the largest wildfires in state history, but residents may not enjoy much
relief as experts said the flames are laying the groundwork for the next
disaster - mudslides.

The intense fire is burning away vegetation that holds the soil in place
and baking a waxy layer into the earth that prevents the water from
sinking more than a few inches into the ground, experts said.

With one heavy rain, the soil above this waterproof layer can become
saturated, start to slide in hilly areas and transform into something
catastrophic.

"Pretty much anywhere there's a fire on a steep slope, there's cause for
concern," Jason Kean, research hydrologist for the U.S. Geological
Survey, said in a telephone interview.

And the Thomas Fire, which has burned 234,000 acres and destroyed nearly
700 homes in Ventura and Santa Barbara counties, is definitely in
landslide country.

“If we get hard rain, there are going to be terrible landslides in the
burn areas," Carla D'Antonio, chairman of University of California,
Santa Barbara's environmental studies program, said in an email.

"It doesn't take a lot of rain to get the soil and rock moving, so to
have burned soil on top of this and no significant plant cover creates
huge potential for landslides," she added.

Among the cities at risk is Santa Barbara, with 92,000 people, as well
as the smaller communities of Carpinteria, Ojai and Summerland.

"It's terrifying," Jamey Geston, 19, of Carpinteria, said of possible
mudslides. "I am just taking it one natural disaster at a time at this
point and try to get through it."

Once the fire is out, more work will begin as officials will likely need
to rush to build retention basins and other structures to prevent debris
flows before the rainy season begins, said Professor Nicholas Pinter of
University of California, Davis' Department of Earth and Planetary
Sciences.

"This is exactly the thing we worry about in the winter following an
event like the Thomas Fire," he said by telephone.

Another large concern is the potential damage to water quality,
Santa Barbara Mayor Helene Schneider said in a telephone interview.

Heavy rainfall could bring lots of silt to waterways like Lake
Cachuma, where barriers are already being erected, as well as
unwanted matter, she said. In 2007, after the massive Zaca Fire,
Santa Barbara spent more than $1 million on extra cleaning and
filtration systems.

The Federal Emergency Management Agency and the state could defray
some costs with grants, but the best outcome would be "a nice, calm,
intermittent rain," Schneider said.

"We don't see any rain in the immediate forecast, which is a curse
and a blessing," she said. "We could use the water to fight the
fire, but we don't want some kind of big downpour that would cause
significant mudslides so soon after the area's been burnt to
nothing."